Pseudomonas aeruginosa MdaB and WrbA are Water-soluble Two-electron Quinone Oxidoreductases with the Potential to Defend against Oxidative Stress

Water-soluble quinone oxidoreductases capable of reducing quinone substrates via a concerted two-electron mechanism have been implicated in bacterial antioxidant defence. Two-electron transfer avoids formation of dangerously reactive semi-quinone intermediates, moreover previous work in Pseudomonas putida indicated a direct protective effect for the quinols generated by an over-expressed oxidoreductase. Here, the Pseudomonas aeruginosa orthologs of five quinone oxidoreductases - MdaB, ChrR, WrbA, NfsB, and NQO1 - were tested for their possible role in defending P. aeruginosa against H2O2 challenge. In in vitro assays, each enzyme was shown to reduce quinone substrates with only minimal semiquinone formation. However, when each was individually over-expressed in P. aeruginosa no overt H2O2-protective phenotype was observed. It was shown that this was due to a masking effect of the P. aeruginosa catalase, KatA; in a katA mutant, H2O2 challenged strains over-expressing the WrbA and MdaB orthologs grew significantly better than the empty plasmid control. A growth advantage was also observed for H2O2 challenged P. putida strains over-expressing P. aeruginosa wrbA, mdaB or katA. Despite not conferring a growth advantage to wild type P. aeruginosa, it is possible that these quinone oxidoreductases defend against H2O2 toxicity at lower concentrations.